Scientists used the degraded strands to reconstruct the entire genetic code of the deadly bacterium - the first time experts have succeeded in drafting the genome of an ancient pathogen, or disease-causing agent.

The researchers found that a specific strain of the plague bug Yersinia pestis caused the pandemic that killed 100 million Europeans - between 30% and 50% of the total population - in just five years between 1347 and 1351.

They also learned that the strain is the "mother" of all modern bubonic plague bacteria.

"Every outbreak across the globe today stems from a descendant of the medieval plague," said lead scientist Dr Hendrik Poinar, from McMaster University in Canada. "With a better understanding of the evolution of this deadly pathogen, we are entering a new era of research into infectious disease."

Although rare, bubonic plague continues to kill some 2,000 people around the world each year.

The scientists, reporting their findings in the journal Nature, analysed the skeletal remains of four individuals exhumed from an East Smithfield "plague pit" sited under what is now the Royal Mint in London.

Tiny scraps of Yersinia pestis DNA were obtained from the victims' dental pulp, and from these fragments, the researchers were able to reconstruct virtually the whole of the bug's genetic code, or genome.

The same techniques could now be used to study the genomes of other ancient pathogens, say experts.

"This will provide us with direct insights into the evolution of human pathogens and historical pandemics," said co-author Dr Johannes Krause, from the University of Tubingen in Germany.